Webster's Revised Unabridged Dictionary · 1913

Dependency

Dependency , noun

1.
State of being dependent; dependence; state of being subordinate; subordination; concatenation; connection; reliance; trust.
Any long series of action, the parts of which have very much dependency each on the other. — Sir J. Reynolds
So that they may acknowledge their dependency on the crown of England. — Bacon
2.
A thing hanging down; a dependence.
3.
That which is attached to something else as its consequence, subordinate, satellite, and the like.
This earth and its dependencies. — T. Burnet
Modes I call such complex ideas which... are considered as dependencies on or affections of substances. — Locke
4.
A territory remote from the kingdom or state to which it belongs, but subject to its dominion; a colony; as, Great Britain has its dependencies in Asia, Africa, and America.

Dependence is more used in the abstract, and dependency in the concrete. The latter is usually restricted in meaning to 3 and 4.